Linux is simple to understand. Typically most Apache and PHP and other packages can be installed via the package-manager that's being used. For RedHat systems (Fedora, CentOS, RedHat) they use YUM and RPM. Debian uses aptitude or apt-get and sometimes synaptic, BSD I believe uses portage (I could be wrong).
But install Apache, with php and mysql is as easy as doing:
yum install httpd php php-* mysql mysql-devel
(Note that php-* will install all php extensions like ming, gd, imagemagick, etc.)
Then all you have to do is know (1) where your httpd.conf file is and (2) where your php.ini is. Then it's a matter of taking a couple hours and setting things up right.
Most servers today would be running a control panel anyway. I used to be a fan of cPanel since you could literally rebuild Apache, PHP and such inside of WHM; however, they use their bundled Apache. I now prefer Plesk since it literally sits on top of the server. So you update Apache and other system files through SSH and Plesk is just a spectator. So there's less difficulty.
If you really want to learn and have the time, I'd build it all by hand. One reason is so you learn about how to build it and if something goes wrong, you can easily fix it (or know how to rebuild it) as well as recompile extensions for it. Secondly, it will be more optimized for your system. Pre-built packages (installed via yum, apt-get, etc.) aren't optimized for your particular processor & chipset. Building from source would allow you to have that extra optimization (if any).
I haven't used Apples Xserve; however, it's a Mac operating system that is built on BSD. So I'm sure the build steps would be extremely close to those for BSD. But, if you don't know much, I wouldn't spend $3000 on a server... I'd rather spend a measley $150 or $400 for a used Dell or new Dell and then wipe it clean and start off that way. Or even just purchase a VPS or dedicated server from somewhere. Then once you know for a fact you can afford it, spend the money on the Xserve.